Lug-to-Lug
Lug-to-lug is the distance between the tips of a watch's top and bottom lugs. It determines how the watch sits on a given wrist size.
Lug-to-lug (often abbreviated L2L) is the straight-line distance from the tip of the top lugs to the tip of the bottom lugs, measured along the case's vertical axis. It tells you how much wrist real estate the watch will actually occupy.
L2L matters more than diameter for fit on a smaller wrist. A 40mm-diameter watch with a 50mm L2L will overhang a 6.5-inch wrist; a 42mm-diameter watch with a 47mm L2L (e.g. the Tudor Black Bay 58) sits cleanly on the same wrist because the lugs don't push beyond the flat top of the wrist bone. As a general guide: L2L should not exceed about 75-80% of your wrist diameter, or the watch will overhang and shift uncomfortably.
When comparison-shopping, two watches with the same diameter can sit dramatically differently — the difference is L2L and lug shape (downward-curved lugs hug the wrist more than straight lugs of the same length).